1. Field of the Invention
This invention relates to a mixer for feeding a gas-fueled engine with a gaseous fuel such as liquefied petroleum gas (LPG) or compressed natural gas (CNG).
2. Description of the Related Art
Any mixer for mixing a gaseous fuel with air prior to the supply to a gas engine has a nozzle which opens into an air inlet path through a venturi in a direction across the axial line of the air inlet path in order to permit an appropriate amount of fuel to enter the air inlet path.
Like vaporizers of gasoline engines and unlike ideal venturi's used for measuring fluid flow rate, a venturi used in an air inlet path of a mixer for a gas engine must be configured to have a large expanding angle at a downstream of the narrowest portion in order to minimize the length of the air inlet path so as to meet the spatial requirements that the outer dimension of the mixer fits well in its mounting space.
Since gaseous fuels, in general, however, are by far bulky than liquid fuels, an enormous volume must be introduced through the nozzle. If the nozzle opens at the narrowest portion of the venturi, then the gaseous fuel, exiting from the nozzle, hits the inlet air flow and causes a large mixture loss. To minimize the mixture loss, ordinary designs make the nozzle to open into a portion of the air inlet path offset from the flow line of the inlet air at a downstream of the narrowest portion (see FIG. 1 of Japanese Patent Post-Examination Publication 49(1974)-9122 and FIG. 1 of Japanese Patent Post-Examination Publication No. 52(1977)-13580). Thus, enlargement of the expanding angle of the venturi contributes both to resolution of the mounting spatial problem of the mixer and to reduction of the mixture loss.
In this arrangement, however, the air, maximized in flow rate at the narrowest portion of the air inlet path, is gradually slowed down in the expanded portion. More specifically, a part of the air in contact with or near the wall of the venturi is slowed down more than the central portion because of its viscosity and hence makes a boundary layer. The boundary layer gradually grows as it runs downstream, because of a further decrease in flow rate, and it produces a ventilation resistance which behaves to stress the flow of the inlet air, i.e. the main flow, running toward the engine. When the boundary layer further grows, the flow rate of the air in contact with and near the wall of the venturi becomes zero, and a further progress of this phenomenon even makes an opposite flow. As a result, the main flow is separated from the wall of the venturi, and the ventilation resistance increases.
Therefore, sufficient enlargement of the expanding angle is not practical with the venturi structure which aims shortening the air inlet path by expanding the venturi at the downstream of its narrowest portion and aims reduction of the mixture loss by opening the fuel nozzle into the air inlet path at the downstream of the narrowest portion, because a large expansion invites a boundary separation, which leads to a decrease in engine output caused by an increase in ventilation resistance.
That is, the prior art mixer has been encumbered with the contradictory problems that an increase in the expanded angle certainly reduces the mixture loss but also increases the ventilation resistance; and a decrease in the expanded angle certainly reduces the ventilation resistance but also increases the mixture loss, and no mixer has been known which uses a venturi structure providing a small mixture loss and a small ventilation resistance.